The present invention relates to composite doctor blades. More particularly, the present invention relates to composite doctor blades for use in papermaking, for example in calenders during the manufacture of printing paper. The term “calender” and variations thereof, as used herein, is intended to refer to an apparatus used to calender paper, including stand-alone calendering units such as supercalenders and calendering units within a papermachine such as machine calenders, gloss calenders and soft nip calenders. The present invention further relates to a method of using doctor blades in calenders.
Doctor blades are widely used to remove various materials from the surface of papermachine rolls. By its very nature, the process of removal of contaminants from the roll surface may result in significant wear to the roll surface, the doctor blade itself or both. The components of paper, particularly coating components, tend to be abrasive and tend to cause wear in the surface of the papermachine roll. Conventional doctor blades may be constructed from metal, e.g., steel, stainless steel, nickel or bronze, metal coated with alloy or ceramic material, plastic, or “composite” materials, i.e., fiber-reinforced polymeric materials. FIG. 1 shows a typical papermachine configuration wherein a doctor blade 2 is positioned against a surface 16 of a papermachine roll 12, for example a calender roll. Doctor blades typically have a 45° beveled edge 14, as shown in FIG. 1.
Metal blades generally exhibit high stiffness in the machine direction, i.e., the direction perpendicular to the rotational axis of the papermachine roll, and good wear characteristics. The machine direction of the papermaking process is generally known in the art as the direction of the paper web as it passes through the papermachine and is indicated by arrow 18 in FIG. 1. Such blades also tend to be susceptible to corrosion and to cause excessive roll wear.
Plastic blades tend to be used in papermachine locations unsuitable for metal blades. Plastic blades, however, generally have significant drawbacks because they tend to have low stiffness and tend to degrade at the temperatures typically used in the papermaking process.
Composite blades are typically formed from a plurality of fibrous layers impregnated with resin, each fibrous layer generally having a woven structure such that a certain proportion of the fibers lay in the machine direction, while the remaining fibers lay in the cross-machine direction, i.e., the direction parallel to the rotational axis of the papermachine roll. The cross-machine direction is generally known is the art as the direction perpendicular to the path of the paper web and is indicated by arrow 20 in FIG. 1. Although composite blades tend to wear more quickly than metal blades, they also tend to cause less wear on the roll surface. Reduced blade life is typically viewed as a drawback and improved wear resistance of the blade is seen as desirable for many doctoring applications. The wear characteristics of composite doctor blades are generally considered acceptable in many conventional calendering applications because excessive roll wear may deleteriously affect the final properties of the paper.
Composite doctor blades are often used with on-line calenders, which are typically run at relatively high nip pressures and high roll surface temperatures. These operating conditions tend to increase the amount of coating particles and contaminants on the calender roll surface. If the calendering rolls are not doctored on an almost continuous basis, buildup of coating particles and contaminants reach unacceptable levels, directly affecting the final product properties of the paper, such as paper gloss and paper smoothness. Moreover, the abrasiveness of the particles and contaminants tend to degrade the surface of the calender roll, causing a permanent degradation of the roll surface. Degradation in the roll surface tends to cause a deterioration of the roll profile, i.e., the roll surface is uneven which tends to cause inconsistent calendering across the width of the paper web. Thus, the demand for consistent paper quality at a high production rate and with greater efficiency has typically resulted in almost continuous doctoring of the calendering rolls during operation to remove contaminants. As a result, there have been significant efforts to increase the wear resistance and, consequently, the blade life of composite doctor blades.
The operating conditions for on-line calendering have also driven efforts to increase the wear resistance of the calendering rolls. It is becoming more common for such on-line calendering rolls to be coated with a thin layer of thermal spray coating, which typically exhibits resistance to roll surface degradation and, consequently, deterioration of the roll profile. The term “thermal spray” and variations thereof, as used herein, is intended to refer to one of three standard processes: plasma, high velocity oxygen-fuel (HVOF), and detonation gun, whereby a material, typically in powder form, is heated and deposited on a surface. The thermal spray coating tends to be a ceramic or metal matrix coating. The surface of a thermal spray coated roll may also be ground to a very low roughness, a highly desirable property for calendering rolls used in the manufacture of coated printing papers.
Thermal spray coatings tend to resist scratching from doctoring activities when such doctoring activities are performed on an intermittent basis, such as the removal of paper wrapped around a roll after a break in the paper web. A thermal spray coated roll will, however, generally exhibit roll degradation when subjected to almost continuous doctoring. Over time, thermal spray coated rolls tend to exhibit deterioration in the roll profile and surface finish caused by the action of the doctor blade and the contaminants. When the roll profile and surface finish have degraded to an extent such that the quality of the paper is adversely affected, the roll must be removed and reground. Removal for grinding can result in a significant loss to production and increased costs. In addition, the grinding process itself removes a valuable layer of thermal spray coating from the roll. Because the thermal spray coating layer of the roll tends to represent a significant portion of the cost of the roll and a significant monetary investment, minimizing the loss of thermal spray coating is highly desirable.
Efforts to increase the wear resistance of composite doctor blades may result in more rapid deterioration of the surface of the roll. On the other hand, an adequate level of wear resistance is required to minimize disruptions to production caused by the need to change doctor blades. There remains a need for a doctor blade that may be used almost continuously against the surface of a thermal spray coated roll to adequately remove surface contaminants, while exhibiting sufficient wear resistance to be practical in the production setting. There also remains a need for a doctor blade that may be used to maintain a low surface roughness of the roll with minimal deterioration of the thermal spray coating.